The present invention relates to a fastener driving tool such as a nail gun, and more particularly, to the nail gun electrically or pneumatically operated for providing a fine appearance on a workpiece after nail fastening.
Attachment members or finishing materials for use in a baseboard for trimming purposes or for a verandah running on more than two sides of a house or room are adhesively fixed to a wall, and then fixed by fasteners such as nails. A groove having a predetermined small width is formed in the attachment member, and the nails are driven into the groove. The nails have a head portion whose color is selected to be in conformance with the external color of the attachment member. Further, the head portion of the nail has a small diameter, so that the head portion can be positioned onto a bottom of the small width groove in an attempt to obtain good external appearance on the attachment member without any scratches as bruises after the nail driving.
A conventional pneumatically operated nail gun includes a main housing, a magazine, a drive bit, a bit guide and a contact arm. A plurality of nails are arrayed side by side in the magazine and a frontmost nail is introduced into the bit guide and is driven into a workpiece such as an attachment member by the drive bit. The contact arm has a free end portion adapted to be in pressure contact with the workpiece. The contact arm has another end portion adapted to push a trigger plate. A valve plunger can be pushed up upon manipulation of a trigger as far as the trigger plate is in the pushed up position. The contact arm is normally biased by a spring toward the attachment member, i.e., in a pushed out direction from the main housing. That is, the contact arm is movable relative to the main housing. When the contact arm is pushed or retracted into the main housing against the biasing force of the spring and the trigger are operated, a nail driving operation can only be started. Therefore, accidental nail shooting other than intended nail driving operation can be prevented.
According to such a conventional nail gun, the nail gun must be moved away from the attachment member so as to separate the free end of the contact arm from the attachment member after the nail driving operation, if the nail gun is to be moved to a next nail shooting position. Because the contact arm is urged toward the pushed out stroke end position by the biasing force of the spring, the nail gun must further be moved by a moving stroke of the contact arm in case of the movement of the nail gun to the next nail shooting position. Here, the moving stroke implies a distance between the most pushed out position (hereinafter simply referred to as a bottom dead center) and the most retracted position (hereinafter simply referred to as an top dead center) of the contact arm with respect to the main housing.
Laid open Japanese Paten Application Publication No. 2002-283253 discloses a nail gun provided with an urging member that normally urges a contact arm toward the top dead center. This can reduce pushing force of the free end of the contact arm against the attachment member for the nail driving operation. Thus, finishing appearance of the attachment member and operability to the nail gun can be improved.
In the nail gun disclosed in the JP publication, immediately after the nail driving operation upon pulling a trigger, a free end of a contact arm is maintained at its bottom dead center position. The contact arm is maintained at a pushed down position by a trigger arm because a plunger of a trigger valve maintains the posture of the trigger arm by way of a spring associated with the plunger as long as a pulling state of the trigger is maintained after the nail driving operation, even if the nail gun is moved to separate from the attachment member. The contact arm restores its top dead center position only when the trigger is released.
A great number of nails must be driven into the attachment member in the nail driving work within a limited period of time. Therefore, an operator must promptly shift the nail gun to a next nail shooting position immediately after the nail driving operation. Normally, the operator maintains pulling state of the trigger during shifting of the nail gun to the next shooting position, and then releases the trigger after the nail gun is placed on the next shooting position. Alternatively, the operator maintains pulling state of the trigger and then releases the trigger during shifting of the nail gun to the next shooting position. In other words, the contact arm maintained at its bottom dead center position during shifting of the nail gun to the next shooting position as long as pulling state of the trigger is maintained. Consequently, the free end of the contact arm may be tangled or abutted against walls of the grooves of the attachment member to damage to the contact arm and the attachment member. If the trigger is released after the nail gun is placed on the next shooting position, the contact arm promptly restores its top dead center position upon release of the trigger. Accordingly, the nail gun must be further moved toward the attachment member by the moving stroke of the contact arm, whereupon moving impact force is generated so that the free end of the contact arm may damage to the attachment member.